The present invention relates to a distributor injection pump for internal combusticn engines, particularly diesel motors.
Known distributor injection pumps of the type under consideration have a hydraulic control chamber which is subdivided by an adjustment piston, actuated by an adjustment member, into two portions which are continually hydraulically pressurized. The adjustment piston has two throttle gaps through which both pressure chambers are in connection with each other and with an initial hydraulic pressure. A slide valve normally provided in such a pump has two control chambers arranged at two end faces of the control slide, which chambers are connected with one of the two aforementioned portions of the hydraulic control chamber. If an exiting voltage is applied to the piezoelectric adjustment member the latter expands and displaces the adjustment piston. Pressure rises in one pressure chamber in a sudden manner and decreases in the other pressure chamber, also in the sudden fashion. Pressure in the control chambers changes in the same manner. The control slide is adjusted by the difference in pressures and the valve opens. If the exiting voltage is no longer applied to the adjustment member the adjustment member is displaced in the opposite direction. Pressure ratios in the pressure chamber portions and thus in the control chambers interchange, and the control slide is again brought to the open position.
The avoidance of the formation of gas bubbles in the pressure medium (diesel oil) of the hydraulic control chamber filled with fuel is obtained by that the pressure chambers are continually hydraulically prestressed. Due to a double-side acting adjustment piston, as opposed to one-side operating pistons, there is no danger that gas would drag in the pressure medium through the guidance gap. In order to overcome the problem of the length change of the piezoelectric adjustment member due to temperature fluctuations and various volume expansions of the pressure medium in the hydraulic control chamber the portions of the pressure chamber are connected to each other by a throttle gap; a hydraulic translation ratio of the cross-section of the piston to the cross-section of the control slide is greater than the ratio between the path of the control slide and the path of the piston, and the adjustment path of the control slide is limited by adjustable mechanical stops.
As studies have shown, despite such construction expenses and extremely quick strokes of the control slide and piezoelectric adjustment member over a longer period of time, the occurence of gas bubbles in the pressure medium of the hydraulic control chamber can not be totally avoided as desired. Gas bubbles in the hydraulic control chamber cause not only changes in the length of the piezoelectric adjustment member due to temperature fluctuations but also to an undesired adjustment of the injection start and/or end and thereby amounts of fuel being injected. Thus the distributor injection pump does not operate optimally.